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https://issues.apache.org/jira/browse/FOP-2847?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Tobias Hain updated FOP-2847:
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Attachment: sample_image_palette_tRNS.png
> Support palette-based transparency via tRNS chunk of PNG images in PDF export
> -----------------------------------------------------------------------------
>
> Key: FOP-2847
> URL: https://issues.apache.org/jira/browse/FOP-2847
> Project: FOP
> Issue Type: Bug
> Components: renderer/pdf
> Affects Versions: 2.3
> Reporter: Tobias Hain
> Priority: Major
> Attachments: sample_image_palette_tRNS.png
>
>
> h4. TL;NR
> When using palette-based transparency via tRNS chunk in PNG images, the
> generated PDFs are broken.
>
> h4. palette-based with transparency.
> {quote}*8.5.2. Palette-Based with Transparency*
> "The PNG spec forbids the use of a full alpha channel with palette-based
> images, but it does allow `cheap alpha` via the transparency chunk, tRNS
> [...] In effect, it transforms the palette from an RGB lookup table to an
> RGBA table [...] a single PNG transparency entry should come first"{quote}
> http://www.libpng.org/pub/png/book/chapter08.html#png.ch08.div.5.2
> h4. general PDF encoding strategy
> Option 1:
> {quote}most libraries just undo the palettization and use a full color image
> + mask.{quote}
> https://forums.adobe.com/message/6148131#6148131
> Option 2:
> {quote}SMask would be simpler since you could leave the palette in
> place{quote}
> https://forums.adobe.com/message/6148557#6148557
> h4. what does FOP do
> It creates a chroma-key mask:
>
> {code}
> /Type /XObject
> /ColorSpace [/Indexed /DeviceRGB 255 <D9DB ...>]
> /Mask [182 182 151 151 158 158]
> {code}
> Acrobat Reader even fails to read those files while PDF viewers embedded into
> Chrome/Firefox read the file, but show visual glitches.
> FOP doesn't implement Option 1 yet. None of the ImageLoaders is prepared to
> do that:
> https://wiki.apache.org/xmlgraphics-fop/HowTo/ImageLoaderRawPNG
> In fact the following results in missing Option 2:
> {code}
> public void setup(PDFDocument doc) {
> RenderedImage ri = getImage().getRenderedImage();
> // code deleted
> Raster raster = GraphicsUtil.getAlphaRaster(ri);
> if (raster != null) {
> AlphaRasterImage alphaImage = new AlphaRasterImage("Mask:" + getKey(),
> raster);
> this.softMask = doc.addImage(null, alphaImage).makeReference();
> }
> {code}
> https://github.com/apache/fop/blob/ff452f3685a02715aee791d651b58dd1797ddfd4/fop-core/src/main/java/org/apache/fop/render/pdf/ImageRenderedAdapter.java
> In the above code raster will be null and therefore FOP fails to initialize a
> softMask. {{GraphicsUtil.getAlphaRaster()}} calls internally
> {{ColorModel.getAlphaRaster()}}, which returns null:
> {quote}If this is an IndexColorModel which has alpha in the lookup table,
> this method will return null since there is no spatially discrete alpha
> channel.{quote}
> https://docs.oracle.com/javase/8/docs/api/index.html?java/awt/image/ColorModel.html
> This code will then add the chroma key mask to the image:
> {code}
> protected void populateXObjectDictionaryForIndexColorModel(PDFDictionary
> dict, IndexColorModel icm) {
> // code deleted
> Integer index = getIndexOfFirstTransparentColorInPalette(icm);
> if (index != null) {
> PDFArray mask = new PDFArray(dict);
> mask.add(index);
> mask.add(index);
> dict.put("Mask", mask);
> }
> }
> {code}
> https://github.com/apache/fop/blob/ff452f3685a02715aee791d651b58dd1797ddfd4/fop-core/src/main/java/org/apache/fop/render/pdf/AbstractImageAdapter.java
> h4. why care to support?
> Libraries such as pngquant optimize images for web usage and low footprint
> and can not be used directly with Apache FOP as source images:
> {quote}"*pngquant*: Lossy PNG optimizer, reducing a PNG image down to an 8
> bit color palette with dithering. It will build indexed-color PNG's with
> alpha transparency colors conveyed in the tRNS chunk."{quote}
> https://imagemagick.org/Usage/formats/#png_non-im
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